Update issue templates

Author: mdo

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,15 @@
+---
+name: Feature request
+about: Suggest an idea for a new feature in Bootstrap.
+
+---
+
+Before opening:
+
+- [Search for duplicate or closed issues](https://github.com/twbs/bootstrap/issues?utf8=%E2%9C%93&q=is%3Aissue)
+- Read the [contributing guidelines](https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md)
+
+Feature requests must include:
+
+- As much detail as possible for what we should add and why it's important to Bootstrap
+- Relevant links to prior art, screenshots, or live demos whenever possible